A reducing agent addition valve may be provided in an exhaust passage of an internal combustion engine in order to add a reducing agent. Particulate matter contained in exhaust gas adheres to the reducing agent addition valve. When a temperature of the reducing agent addition valve is high, the particulate matter adhered to the reducing agent addition valve is hardened by heat, and as a result, a nozzle hole of the reducing agent injection valve may be blocked. In other words, a blockage may occur in the reducing agent addition valve. Such a blockage can be prevented from occurring in the reducing agent addition valve by reducing the temperature of the reducing agent addition valve, and the temperature of the reducing agent addition valve can be reduced by periodically injecting the reducing agent from the reducing agent addition valve. Reducing agent addition implemented in this manner to prevent, a reducing agent blockage will be referred to hereafter as a “blockage prevention injection”.
In a conventional technique, a fuel addition valve that injects fuel and an oxidation catalyst are provided in an exhaust passage of an internal combustion engine in sequence from an upstream side, and an amount of fuel added from the fuel addition valve during a blockage prevention injection is limited prior to activation of-the oxidation catalyst (see Patent Document 1, for example).
Incidentally, a selective reduction type NOX catalyst (also referred to hereafter as an SCR catalyst) may be provided on a downstream side of the oxidation catalyst. An NOX purification ratio of the SCR catalyst may vary according to a proportion of NO2 within an amount of NOX contained in the exhaust gas that flows into the SCR catalyst (referred to hereafter as an NO2 ratio). Here, when a blockage prevention injection is implemented, an oxidation capacity of the oxidation catalyst decreases. Accordingly, oxidation from NO into NO2 is suppressed, and as a result, the NO2 ratio decreases, When the NO2 ratio decreases in this manner, the NOX purification ratio of the SCR catalyst may also decrease. In other words, the NOX purification ratio may decrease as a result of the blockage prevention injection.